To come into gomukhasana, begin seated with outstretched legs (dandasana). Bend the right knee, placing the foot on the floor. Bend the left knee and tuck it under the right leg on the floor. The knee should be directly in front of you with the left foot facing behind you. If you cannot bring the leg to this position, work on the hips little by little, and consider supplementing your practice with a variety of hip openers. Fold the right leg onto the left so the knees are stacked and both feet are facing the wall behind you. Keep both sitting bones connected to the floor the entire time. The pelvis should be neutral and grounded, with the spine lifting up from the hips. If you cannot get both sitting bones on the floor, spread the knees a bit, and again, consider adding more hip openers to your practice.
Once the legs are set up you can move up to the arms. A strap is a very useful prop here. If you don't have a strap a necktie or t-shirt can suffice. Lift the left arm straight above your head, perpendicular to the floor (holding the strap if you are using one) and bend at the elbow. Bend the right elbow behind you (see the above figures) with the palm facing away from the back. The elbow should be facing the floor, but if it does not completely face the floor, do not force the stretch. Work from where you are and let the muscles open up in their own time. Grab the strap with the right hand, then walk the hands toward each other up the strap. In the full variation of this posture, the hands clasp each other. If you are not there yet, work the hands closer and closer each time you perform this pose. If you don't have a strap or prop, face both palms toward the body and "crawl" the fingers up the back, trying to get the hands closer to each other each time. Close your eyes and notice the sensations in the back and shoulders. Hold this pose for 8-10 breaths, then switch sides.
Gomukhasana is a wonderful pose for stretching out the upper and lower body and leaves you feeling exhilarated and accomplished. Namaste.